As well known to those skilled in the art, it is impossible to vary valve lift or opening duration using typical cams provided in conventional engine systems. Furthermore, because the valve lift and opening duration are fixed as specific values, a fuel consumption ratio and power output of an engine cannot be optimized.
In an effort to overcome the problems experienced with the conventional arts, recently, studies have been conduced on variation of opening and shutting times and periods of intake and exhaust valves in order to increase thermal efficiency and power output of engines. As part of these studies, a continuously variable valve lift system has been proposed.
In detail, the continuously variable valve lift system is able to adjust opening and shutting times of intake and exhaust valves and factors such as valve lift, related to valve movement, such that they are optimized. For example, the system maximizes the inflow rate of the engine in a high-speed or high-loading condition that requires high output power. In a low-speed or low-loading condition in which it is important to increase a fuel consumption ratio or reduce exhaust gas, the system increases an EGR (exhaust gas recirculation) effect and minimizes throttle loss.
However, in the conventional continuously variable valve lift system, structures of moving parts are complex. Furthermore, because the system requires a large space above a position at which a cam shaft is mounted in the cylinder head of the engine, the overall height of the cylinder head is increased.
As a result, the volume of an engine having the conventional continuously variable valve lift system is increased. As well, due to an increase in space required for the cylinder head in an engine room, there is a spatial restriction in the installation of other elements in the engine room.